x86, mce: unify, prepare 64bit in mce.h

Prepare mce.h for unification, so that it will build on 32-bit x86
kernels too.

[ Impact: cleanup ]

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>